I have read that the rate of reproduction in blackworms is fairly slow and you need a decent amount of them to begin with if you are going to be regularly harvesting them for your fish.

To keep blackworms alive, all you have to do is place them in a container and then add enough water that it just covers them. Don't add too much or they can drown and will start to die off.

The container should be kept in the fridge and I usually completely change the water every one or two days. Mine can live for up to a month like that (usually my fish eat them all before then) and it has been the best method I have found so far that doesn't require a lot of effort.